The present invention relates to a voltage standing-wave ratio (VSWR) measurement apparatus. The present invention also relates to an apparatus for, and method of, monitoring a cellular communications system, particularly the radio transmission lines and antennas of such a system. The present invention is applicable to, but not limited to, cellular radio communication systems such as the Global System for Mobile communications (GSM), and also the Universal Mobile Telecommunication System (UMTS) currently under standardisation. The present invention is applicable to, but not limited to, picocellular arrangements.
One type of communications system is a cellular communications system. In a cellular communications system, the area over which service is provided is divided into a number of smaller areas called cells. Typically each cell is served from a base transceiver station (BTS) which has a corresponding antenna or antennas for transmission to and reception from a user station, normally a mobile station. Presently established cellular radio communications systems include GSM systems (Global System for Mobile Communications).
Cellular communications systems can also include sub-cells arranged within the above mentioned cells. Such cells within cells are often called microcells. When different areas within a building or series of buildings are divided into different sub-cells, for example by having a radio transceiver on each floor providing a respective sub-cell, the sub-cells of such an arrangement are often called picocells.
In a typical arrangement of a cellular system, the antenna or antennas corresponding to a BTS are connected to said BTS via respective links, at least part of such a link being a radio link, for example in the form of a radio frequency (RF) radio transmission line. This will typically be implemented in the form of co-axial or otherwise screened cable, plus connections being required along such cable and at interfaces into other equipment as required by the practical layout arising.
A known method of monitoring transmission lines/antennas is to measure voltage standing-wave ratio (VSWR). The power reflected back along a transmission line from an antenna is compared to the power being sent along the transmission line to the antenna. In the case of an undamaged, correctly matched transmission line, antenna, combination there would only be a very low proportion of power reflected back, because the antenna would radiate the majority of power fed to it.
A disadvantage with carrying out VSWR monitoring, especially when there is a significant length of cable between the measurement point and the terminating antenna(s), as is often the case in cellular communications systems, has been found to be that losses in cables plus losses at connections can themselves account for a reduction in reflected power to an extent that even when an antenna is in error the monitoring method sees the result as acceptable. Furthermore, by their nature, cellular communications systems often have a large number of discontinuities in their radio transmission lines. The term discontinuity is used to describe points along a radio transmission line where the radio transmission line is split into more than one transmission line, for example so as to accommodate more than one antenna from a given radio transceiver unit. The term discontinuity also extends to any point along a radio transmission line where due to connections to specific equipment or similar arrangements it can be predicted, or it is discovered, that the characteristics of the radio transmission line are, or tend to be, affected or altered at that point. Yet further, the term discontinuity also extends to connections or joints between lengths of cable or patch panels. These discontinuities can result in mismatches in the radio transmission lines that reflect power back towards the transmission source and can cause remote VSWR measuring equipment to show a poor VSWR when the antennas are actually radiating satisfactorily, and additionally can mask the VSWR measurement of a antenna that is actually faulty or poorly matched.
The above disadvantages are particularly prevalent in a picocellular system where a large number of antennas are distributed in a building, resulting in a large number of such discontinuities.
The present invention addresses some or all of the above disadvantages by providing apparatus for detecting a fault in a system such as a cellular communications system and which apparatus includes voltage standing-wave ratio (VSWR) measurement apparatus.
The present invention advantageously allows for sensitive monitoring at different points along a radio transmission line, to a sensitivity level improved over what would be the case if only the whole link could be monitored as one item. Also, the present invention usefully indicates the location within the system of an irregular VSWR measurement. This is particularly useful in systems with distributed antennas, such as a system incorporating an in-building picocellular arrangement. The present invention also facilitates different threshold settings at different locations, accommodating different contributions of reflective power at different stages of a radio transmission line.
Additional specific advantages are apparent from the following description and figures which relate to a merely exemplary embodiment of the present invention.